Marine surveys typically illuminate a subterranean formation located beneath a body of water with acoustic signals produced by one or more submerged impulsive sources. A typical impulsive source includes an array of air guns, each of which is connected by a hose to a compressor located onboard a survey vessel that tows the impulsive source through the body of water. Each air gun has a chamber that stores compressed air or gas at a selected charge pressure. An impulsive source may be activated by electronically triggering the air guns. When an air gun is triggered, air or gas is forced through vents into the water, creating a high-pressure, oscillating primary bubble followed by a foam of smaller oscillating secondary bubbles. The bubble oscillation period of a primary bubble produced by a single air gun may be related to the volume of the air gun chamber and charge pressure of the air or gas stored in the chamber as follows:
                    T        =                  K          ⁢                                                    P                                  1                  ⁢                                      /                                    ⁢                  3                                            ⁢                              V                                  1                  ⁢                                      /                                    ⁢                  3                                                                                    (                                                      P                    0                                    +                                      ρ                    ⁢                                                                                  ⁢                    gD                                                  )                                            5                ⁢                                  /                                ⁢                6                                                                        (        1        )            
where                T is a bubble oscillation period;        P is the air or gas charge pressure of the air gun;        V is the air gun chamber volume;        P0 is atmospheric pressure;        ρ is density of water;        g is Earth's gravitational acceleration;        D is depth of the air gun in the water; and        K is a constant that depends on the units of measure of the forgoing parameters.The vibrational frequency of the primary bubble oscillation is f=1/T. An oscillating primary bubble creates acoustic energy at vibrational frequencies that allow the acoustic energy to propagate into the subterranean formation.        
The air guns of an array may be selected with particular chamber volumes and operated at selected charge pressures so that when the air guns are activated nearly simultaneously a desired acoustic signal is produced. The acoustic signal vibrates at frequencies that allow the acoustic energy to propagate into the subterranean formation. According to Equation (1), the lowest frequency bubble oscillation of an air gun array, flow, is the reciprocal of the longest bubble oscillation period, Tlongest, which is produced by the air gun with the largest chamber volume and/or highest charge pressure.
Although air gun arrays are widely used in marine surveys, air gun arrays are typically not configured with air guns that generate acoustic energy at frequencies below about 8 Hz. Equation (1) indicates that it should be theoretically possible to increase the bubble period T (i.e., expand the low-frequency end of an air gun array frequency spectrum) by simply increasing the chamber volume of the largest air guns and/or operating certain air guns at higher charge pressures. While it may be theoretically possible to build such air guns, the size of the air guns needed to support larger chamber volumes may be impractical and would require a substantial increase in the capacity of a shipboard compressor used to fill the chamber. Air guns also become increasingly less reliable when operated at higher charge pressures. Those working in marine seismology continue to seek methods and systems to expand the low-frequency range of acoustic signals used to illuminate a subterranean formation.